The term four-wheel drive is often thought to mean that power goes to all four wheels. This is often not the case, as the differentials sit where the drive shafts meet the axles.
As the name implies, differentials allow for the tyres to move at different speeds. This difference is used when turning the vehicle, as the outside tyre travels further than the inside. This is bad when you are trying to power through an obstacle. This is when locking differentials ( lockers ) come into play. A locker locks the differentials and put power to both wheels.
How Lockers Work
Open diffs transfer power to tyres with the least resistance, usually the one spinning in the air. The differential not only changes the direction that the power is traveling but also multiplies the power by the gear ratio of the ring and pinion.
A locker allows the locked diff to transfer power to both tires on a given axle regardless of the traction on the two sides. Lockers make a huge difference off-road because they double your traction by spinning both tires.
If you hang a tire in the air with an open differential, it will just spin while the other tire sits still. Adding a locking differential means that both tires turn at the same speed, regardless of the situation. As a result, lockers are more important in vehicles with limited suspension articulation that regularly have a tire off the ground.
Automatic lockers operate with no input from the driver. Automatic lockers can unlock when one wheel is required to spin faster than the other. They include both full replacement case lockers or ones that replace the side gears within your existing carrier.
While automatic lockers provide excellent traction in all circumstances, there are some quirks to be aware of. Tire wear isn’t as much of a problem as with a spool, but it is still accelerated when compared to an open differential. While accelerating through a turn, a locker can reengage harshly. The result is chirped tires and a vehicle that wants to switch lanes. This is more of an issue when the vehicle has a short wheelbase, high horsepower, and a soft suspension.
Selectable lockers allows the driver to lock and unlock the differential from the driver’s seat. Selectable lockers negate many of the drawbacks that automatic lockers exhibit on the pavement. The differential can act as an open one for better street manners and less tire wear. It also has full locking capability for ultimate traction whenever it is desirable.
Aftermarket selectable lockers can activate via air (such as the ARB Air Locker), electricity (Eaton’s E-Locker), or a cable (Ox Locker). When locked, they operate just like a spool placing a significant load on the axle shafts. But unlike a spool or automatic locker, you have the control to only use a selectable locker when you deem it necessary. This reduces the strain on the drivetrain the rest of the time.
All selectable lockers replace the entire carrier, so they are incredibly durable. Strength and durability aren’t necessarily synonymous with reliability, though. The added complexity of solenoids, switches, air lines, and wires means that there are more moving parts in selectable lockers than automatic lockers. They rarely break, but they can malfunction, mainly if they are installed incorrectly. In the case of Air Lockers, you need to also factor in buying onboard air to activate the locker.
Even with both tires on the ground, there are situations when a locking differential is required to keep you moving forward, particularly in the rocks. Note that in a situation like this, where a tire might be jammed under an obstacle, a locker will place additional stress on the axle shafts and U-joints. Upgrading these components when you add a locking differential is a wise investment.
Front or Rear First?
Decided you want to get lockers but only have budget for one locker, you can read about choosing which locker to get first here.